Factors in Evolution 33 



others often begin to flower in the autumn and con- 

 tinue flowering for several months, and autumn flow- 

 ering plants like the chrysanthemum may prolong 

 their flowering period until February or March. 



How far these changes are intensified by long 

 cultivation in a new environment, has not been criti- 

 cally studied, and of course if the plants were re- 

 stored to the colder climates from which they came, 

 they would have to revert to their original habits 

 or perish, as their changed habits of growth would 

 be quite impossible in their original habitat. 



THE CONDITIONS FOR PLANT GROWTH 



Of the external factors that govern the life of 

 normal green plants the most important are light, 

 heat, gravity, water, oxygen, and various food con- 

 stituents, including CO 2 . 



Light. As all green plants are dependent upon 

 light for their existence, it is quite comprehensible 

 that light has evidently exercised a very powerful 

 formative action upon plant structures. The form 

 of each individual plant is markedly influenced by 

 the character of the light to which it is exposed; 

 a fact readily verified by the most casual observa- 

 tion. The bending of plants towards the light, the 

 weak spindling growth where the light is insufficient, 

 and the complete suppression of chlorophyll and 

 the reduction in the size of the leaves, which are 

 such common phenomena where light is excluded, 



